The use of specialized ink formulations to form thick films having various functions on suitable substrates in the construction of multilayer integrated circuits is known in the art. Such technology is of increasing interest in the fabrication of very dense multilayer circuit patterns on various substrates for a wide variety of applications in the electronics industry.
Thick-film multilayer structures typically are comprised of at least two patterned layers of a conductor separated by a dielectric layer. The patterned conductor layers are connected by a metallic conductor deposited in vias in the dielectric layer. Such structures are formed by multiple deposition and firing of layers of conductor and dielectric inks.
Such multilayer circuit structures utilizing copper as the conductor metal have a number of problems. The most common is failure caused by the development of electrical shorts due to interactions between materials of the copper conductor ink and the dielectric layer which take place during the multiple firings necessary to fabricate multilayer integrated circuits. If the dielectric material is not resistant to the penetration of flux components, conductive channels may be formed in the dielectric during the repeated firings. When such a channel passes completely through a dielectric layer and makes contact between an overlying and underlying copper conductor, an electrical short is produced.
A second problem common to multilayer circuits is porosity in the fired dielectric layer resulting from the evolution of gases from organic vehicle materials and/or oxides of bismuth and copper during firing. Contaminant materials, e.g. molten eutectic phases from fired copper conductor layers, can readily leach into the resulting passages. The integrity of the dielectric layers in a multilayer structure is also important because the voids therein lower the resistance of the dielectric, which is undesirable. It is conventional to print and fire at least two layers of dielectric ink between conductors to minimize the possibility that evolving gases will form connected passages through the dielectric.
It is possible to reduce the tendency of contaminant materials to leach into a dielectric material by formulating the ink with a higher quantity of glass frit thereby decreasing the porosity of the fired material. However, both this solution and the multiple print and fire approach can result in the trapping of gases within the dielectric layer. This will cause both the dielectric layer and overlying conductor layers to blister and peel during subsequent nitrogen firings.
Typically, the glass phase of a glass-filled dielectric will flow and densify at between 500.degree.-650.degree. C. Ideally, all traces of the organic vehicle in the ink should be removed before the glass flows and densifies. Conventionally, this is not readily achieved without the addition of oxidizer constituents such as barium nitrate. While such additives are effective in removing trace amounts of carbonaceous material, they are known to evolve gas, particularly oxygen, during subsequent firings. These gases can cause blistering and peeling of subsequently applied layers and can substantially increase the porosity of overlying dielectric layers. In addition, the gases evolved from these materials can react with copper from adjacent copper conductors and via fills to form copper oxide which, in turn, can react with flux materials and form an eutectic phase which will readily penetrate porous dielectric material.
Another approach to ensure removal of carbonaceous residues from a dielectric material is to treat the dried ink, before firing, with an oxidizing or reducing plasma as disclosed by Prabhu et al. in U.S. Pat. No. 4,619,836, issued Oct. 28, 1986. This treatment, while a significant improvement, does introduce an additional processing step and additional apparatus requirements into the manufacture of copper-based multilayer circuitry.
In accordance with this invention, there are provided novel dielectric inks which form dielectric layers having reduced porosity without oxidizers, such as barium nitrate, or a plasma treatment prior to firing.